1. Field of the Invention
The invention relates to a method for identifying a fast speed mobile station in a radio system, said method comprising measuring the frequency error of radio signals received from a mobile station.
The invention further relates to a base station of a radio system, said base station comprising receiver means for receiving signals transmitted by a radio unit, and measuring means for measuring the frequency error of the received signals.
2. Description of Related Art
The invention relates particularly to the problem created in cellular radio systems when a fast speed mobile station (FSMS) roams in a microcellular network and thus changes base station frequently. Changes of base station load the network management resources significantly and, moreover, the quality of the connection deteriorates. In a situation of this kind it is preferable to hand a fast speed mobile station over to a larger radio cell which serves a larger area "above" the microcells, i.e. which covers the same geographical area as a number of microcells together. The problem that arises is that it is very difficult to identify a fast speed mobile station reliably.
Previously known methods for identifying a fast speed mobile station are based on measuring time. These methods comprise measuring the time between two handovers. If the measured time is sufficiently short, it is assumed that a fast speed mobile station is in question. Since the identification methods are based on measuring the time, a fast speed mobile station has usually reached the area of at least the second or third microcell in a microcellular network before it is identified; the identification thus takes place at a rather late stage. In addition, a mobile station moving on the periphery of a cell can be interpreted as a fast speed mobile station if it moves in an unfavourable direction through the periphery of cells. Since the above-mentioned larger radio cell has usually limited capacity, the interpretation of slow speed mobile stations as fast speed mobile stations may unnecessarily overload the larger radio cell.